The invention relates to impact resistant laminated windows and their manufacture. In particular, the invention relates to a method of producing a pre-product glass sheet from a tempered or non-tempered glass sheet substrate, which, acting as a component which increases the impact strength of a laminated glass window having at least one synthetic resin bonding layer, is adapted to be integrated into the laminated glass window. The invention also relates to a laminated glass window comprising at least one pre-product glass sheet made by the method.
In this specification the term "impact strength" denotes the strength with respect to pulsating, striking or knocking stresses. These include impacts by bullets stones or other hard objects on a window. In the context of the invention, the term also covers bending stresses and, more particularly, locally limited bending stresses which take place at high speeds of deformation and result from a large bending pulse. These includes stresses which aircraft windows are subjected to on collision with birds. In the context of the invention the term "synthetic resin" is not restricted to the definition of DIN 55958 (1988). It also includes the plastics listed in the description and in the claims and other plastics usable in the context of the invention (cf. Rompp "Chemie Lexikon" 1990, 2398 ff.).
It is known from DE-A-2223316 (and its equivalent GB-A-1359165) to subject the surface of at least one of the glass sheets in a laminated glass window to what is known as. rectification, to eliminate surface faults and inhomogeneity and thus improve the strength and, more particularly, the impact strength of the glass sheet. For this purpose it has also been proposed to carry out an- etching operation on at least one of the surfaces. Tests have shown that there is no guarantee of any appreciable reproducible improvement in strength, and particularly impact strength, of the laminated glass window.
In order to increase the bending strength of sheet glass it is also known (CH-PS 367 942, US 37 11 263) to etch away one or both surfaces of a non-tempered or tempered glass sheet by means of an etching liquid, the etch depth having for example a thickness of 30 .mu.m or more. The specifications disclose the provision of the etched surface or surfaces with a thin protective inorganic layer, e.g. a silica protective layer. The Swiss specification discloses that optimal strengthening may be achieved by thermal treatment of the glass directly after etching and before provision of the inorganic protective layer. This protective layer is applied in order to improve the resistance of the glass to sudden temperature changes and to improve the tensile strength. The protective layer may be replaced by a thin glass layer. An improvement in strength is alleged to be obtained in this way. Tests, however, show that even these known steps do not give a reproducible improvement in impact strength. This is because these known processes give a wide variation from sheet to sheet of the strengths obtained.